The invention relates to a process for the preparation of 4-aminodiphenylamine (4-ADPA) by hydrogenation of nitrosobenzene with hydrogen in the presence of hydrogenation catalysts and fluorides.
4-ADPA is an important intermediate for antioxidants and stabilisers in the rubber and polymer industry (Kirk-Othmer, Encyclopedia of Chemical Technology, 4th Edition, 1992, Vol. 3, p 424-447 and p 448-456; Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Vol. A3, 1985, p 91-111).
4-Aminodiphenylamine may be prepared by various methods. One possible route to 4-ADPA is the two-stage (intermediate: 4-nitrodiphenylamine) reaction of aniline or aniline derivatives with p-nitrochlorobenzene in the presence of an acid acceptor or a neutralising agent and optionally in the presence of a catalyst. The preparation by this method is described, for example, in DE-A 35 01 698, DE-A 18 56 63, U.S. Pat. No. 4,670,595, U.S. Pat. No. 4,187,249, U.S. Pat. No. 4,468,333 and U.S. Pat. No. 4,187,248. A disadvantage of such a process is that the salts thereby obtained have to be disposed of at considerable cost. Consequently, aniline or corresponding aniline derivatives have been reacted with nitrobenzene in the presence of tetraalkylammonium hydroxides and in the presence of controlled quantities of protic materials. 4-ADPA was obtained in a satisfactory quantity in this case (see WO 95/00 324 and WO 93 24 450). According to U.S. Pat. No. 5,420,354, 4-ADPA may be obtained by reaction of aniline, nitrobenzene and hydrogen in the presence of a hydrogenation catalyst, hydrogenation inhibitor and acid cocatalyst, albeit in rather unsatisfactory yields. U.S. Pat. No. 5,574,187 describes a method for obtaining 4-ADPA by reacting aniline with nitrosobenzene or phenylhydroxylamine in the presence of acids.
These processes have the disadvantage, however, that two different starting products are used which have to be prepared in upstream, separate process stages, this being rather uneconomic.
It is also known that the hydrogenation of nitrosobenzene by way of heterogeneous catalysts yields chiefly aniline and hydrazobenzene. 4-ADPA is not mentioned as a product (Chem. Ind. 1994, Catalysis of Organic Reactions, p 137-149).
Surprisingly, it has now been found that 4-ADPA may be obtained in industrially useful yields by hydrogenation of nitrosobenzene in the presence of fluorides and heterogeneous catalysts.
The present invention therefore provides a process for the preparation of 4-aminodiphenylamine which is characterised in that nitrosobenzene or mixtures of nitrosobenzene and nitrobenzene are hydrogenated with hydrogen in the presence of fluorides and heterogeneous catalysts and in the presence of inert aprotic solvents at temperatures from 0 to 200.degree. C. and pressures from 0.1 to 150 bar.
Fluorides suitable for the process according to the invention are inorganic fluorides, such as alkali metal fluorides, alkaline earth metal fluorides and the corresponding fluorides of the elements 58 to 71 of the periodic system of elements (according to IUPAC, new). Examples include: the fluorides of sodium, potassium, lithium, caesium, rubidium, magnesium, calcium, barium, lanthanum and/or cerium, particularly the fluorides of lithium, sodium, potassium, caesium, more particularly preferably potassium fluoride, caesium fluoride, sodium fluoride.